The causticizing process has conventionally used a lot of different process steps for;                reception of the green liquor;        separation of dregs from green liquor;        washing and drying dregs obtained from the previous separation step;        mixing of clear green liquor and burnt lime in order to slake lime and start the causticizing reaction;        tanks for completion of the causticizing reaction;        separation of lime mud from white liquor;        lime mud washing and drying.        
A typical conventional causticizing process is shown in FIG. 1. The raw green liquor RGL is first received in an equalizing tank EQT and from there pumped to a first green liquor separation process, here shown as a green liquor pressurized disc filter GLF. The green liquor filter separates dregs from the raw green liquor and produces clear green liquor which is sent to a green liquor storage tank GLT. The clear green liquor is then sent, most often via a green liquor cooler GLC, to the slaker SL where burnt lime is mixed into the green liquor. The cooler is needed to reduce temperature ahead of the slaker to keep the slurry in the slaker under boiling point as the reactions occurring in and after the slaker are exothermic. Grits, i.e. unreacted fractions of the burnt lime, are also separated out from the slaker. After mixing in the slaker, the slurry is sent to a series of causticizing vessels CT1-CT2-CT3, often named the causticizing train, wherein the chemical causticizing reactions are completed. Once these causticizing reactions are completed, the slurry is pumped to a white liquor separation process, here shown as white liquor pressurized disc filter WLF. The white liquor filter separates lime mud from the caustiziced liquor and produces clear white liquor, which is sent to a white liquor storage tank WLT. The clear white liquor is then sent directly to be used in the kraft cooking or bleaching line, or alternatively via a polysulfide modification process to said kraft cooking. The lime mud, which still may have a residual content of alkali, is sent to a lime mud washing and drying stage, here shown as a lime mud pressurized disc filter LMF.
Once the lime mud is washed and dried it may be passed to the lime kiln in order to convert it to burnt lime to be used in the slaker again.
In these conventional causticizing processes as shown in FIG. 1, a specific start up procedure for the green liquor separation process has been used. During start up, the green liquor filter has initially been filled with causticizised liquor from the causticizing train CT1-CT2-CT3 in order to build up a precoat of lime mud on the surface of the filter cloth. The reason for this formation of lime mud precoat is that this precoat exhibit a far better separation efficiency than the cloth itself and has a better filterability than would a precoat formed by dregs from green liquor. The filterability improves by a factor of 6 if a precoat is formed by lime mud instead of green liquor mud (dregs). However, this short establishment of the precoat using causticizised liquor from the causticizing train CT1-CT2-CT3 has never been used for longer periods than about 5% of the total cycle time of the green liquor filter, and as soon as this precoat has been formed, the major part of the operating time for the green liquor filter has been used for green liquor filtering, and the main part of the white liquor produced, typically more than 90% of the total amount, is obtained from the dedicated white liquor filter.
However, usage of pressurized disc filters, one for white liquor filtration and one for green liquor filtration, are expensive as the costs for these filters are high. Filtering techniques are often better as cleaner product liquors could be obtained with small amounts of suspended solids in the product liquors, typically with content less than 20 ppm, as compared with typical green liquor having more than 1500 ppm. Another advantage is that dregs or lime mud separated from these filters could be obtained at very high dryness in the range 40-60% and 60-75% respectively. Alternative techniques has therefore been considered, and usage of conventional settling tanks for green liquor has once again been considered simply due to less investment costs, even though the amount of suspended solids often are much higher, typically four times more.
Another problem with these conventional processes is that so many different and dedicated separation apparatuses are needed, requiring a lot of free building area. This will be problematic when trying to increase capacity of the causticizing plant, as most often no available room is at hand for additional apparatuses increasing the capacity.